Employment Income Tax Calculator in Ruby

taxCalc.rb

class Tax
  attr_accessor :threshold

  def initialize(threshold)
    # There's probably a better way of doing default values
    @threshold = threshold.to_i || 7592
  end

  def parseTaxCode(taxCode)
    # Try this bit in ruby 1.9
=begin
    unless taxCode.ascii_only?
      return false
    end
=end
  
    firstChar = taxCode[0,1]
    secondChar = taxCode[1,1]
    lastChar = taxCode[-1,1]
    length = taxCode.length - 1
  
    # All these type casts are going to be expensive, there must be a better way of doing this
    # but I'm not sure I fully understand the logic.
    case firstChar
    when 'K'
      code = taxCode[1..8].to_i + 1
      code = code.to_s + '0'
      code = code.to_i * -1
    when 'B'
      code = 0
    when 'D'
      code = secondChar === '0' ? 'D0' : 'D1'
    else 
      code = taxCode[0,length]
      code = code.to_s + '0'
      code = code.to_i
    end
  end

  def taxCalc (annualIncome, code)
    nicable = annualIncome - @threshold
    
    if nicable <= 0
      nic = 0
    elsif nicable <= 34892
      nic = nicable * 0.12
    elsif nicable > 34892
      nic = (nicable - 34892) * 0.02 + 4187.04;
    end
  
    # Better would be if code is *parseable to* an integer. (`if !!code.to_i` ?)
    if code.is_a? Integer
      taxable = annualIncome - code.to_i
      if taxable <= 0
        tax = 0
      elsif taxable <= 34370
        tax = taxable * 0.2
      elsif taxable <= 150000
        tax = (taxable - 34370)*0.4 + 6874
      elsif taxable > 150000
        tax = (taxable - 150000)*0.5 + 53126;
      end
    elsif code == 'D0'
      tax = annualIncome * 0.4
    elsif code == 'D1'
      tax = annualIncome * 0.5    
    end
  
    deductions = tax + nic
  
    net = annualIncome - deductions
    
=begin
    # This is for ruby < 1.9
    puts (net/12).round() / 100.0
    puts (tax/12).round() / 100.0
    puts (nic/12).round() / 100.0
=end

    netmonthly = (net/12).round(2).to_s
  
    taxmonthly = (tax/12).round(2).to_s
  
    nicmonthly = (nic/12).round(2).to_s

    # String interpolation is always going to be more performant than concatenation.
    # Even if Ruby's strings are mutable (which I don't know, but would assume so)
    'Your monthly income will be #{netmonthly} and you will have paid #{taxmonthly} in tax and #{nicmonthly} in NI.'
  end

  def run()
    puts 'Hi there, welcome to my Income Tax Calculator, built in Ruby\n'

    puts "What is your annual income?"
    # Is Ruby strongly-typed or what? This language makes no sense :p
    income = gets.chomp.to_i

    puts "What is your tax code?"
    taxCode = gets.chomp

    taxCalc(income, parseTaxCode(taxCode.upcase))
  end
end

# If the constructor value were editable, theoretically the program could be
# kept up-to-date every year automatically. Unless tax works even more weirdly
# than I suspect.
tax = Tax.new(7592)
puts tax.run()

1. I don't know anything about tax codes but I'm pretty sure if I did I could optimise the parser
2. ascii_only? should work but I haven't bothered to upgrade to ruby 1.9
3. Needs validation.

See also: methods & return values (http://docs.oracle.com/javase/tutorial/java/javaOO/returnvalue.html) - Ruby does implicit returns but the principle is the same, it's just less obvious what you're returning. (Hence: ruby is a bad language to learn :P)